2 Answers
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The answer to this will partially depend on how many guns you carry, what they are, and what difficulty level you're playing on, and whether or not you primarily deal damage through biotics and tech powers. Enemies on higher difficulties take a lot more time to whittle down, resulting in higher ammo usage if you're playing, say, a Soldier, who relies mainly on weapons for damage. If the only weapon you carry is a sniper rifle, you'll probably find yourself quickly out of ammo.

Also, when you pick up ammo, it gives you back ammo for every weapon you are carrying. If you're carrying 5 guns, picking up 1 ammo pack is giving you back ammo for each of those 5 guns. This means that so long as you switch weapons, you can keep going in an extended battle for a lot longer than someone with just one or two guns could, since you've got 5 weapons worth of ammo to cycle through..

That said, I'm playing on Insanity and primarily use just an assault rifle and sniper rifle and haven't had too many ammo issues so far. The sniper rifle runs out quickly, but the enemies are usually dead before the assault rifle runs out, and ammo is almost always just around the next corner after a fight.